Continuing our discussion on various causes of wrinkles in overlamination, this month's topic focuses on machine direction wrinkles. Remember our goal of this five part series is to help us all understand the factors that may play a role in the formation of wrinkles throughout the value stream; including film production, adhesive coating & converting and even lamination.
Machine Direction Wrinkles:
This month we’ll discuss machine direction wrinkles. These types of wrinkles are not typically visible in the coated rolls supplied by acpo. These wrinkles are more typically observed in uncoated film rolls. They are relevant to our discussion because the film deformation that they cause can contribute to problems further down the value stream.
The wrinkles that are most often seen are typically MD (machine-direction) wrinkle problems (sometimes called “tin canning”). “Tin-canning” may not be immediately visible during or after winding. Entrapped air between layers of film may temporarily mask this defect. As the boundary air escapes over time the wraps begin to fold into the traditional MD wrinkles or “tin canning” pattern. These MD wrinkles are: generally caused by persistent MD gauge bands in the roll, along with the use of a lay-on roll. This is particularly relevant for thin-gauge films.
The gauge band areas wind at higher radial pressure and carry higher winding tension than the rest of the web. The roll diameter increases in these locations more rapidly than the rest of the roll causing these areas to also carry most of the lay-on contact load. The continued uneven build up of these location on the roll leads to the deformation of the lay-on roll elastomer covering versus the rest of the roll. When this deformation becomes of sufficient magnitude, the film web is actually stretched over the gauge band areas in the CD (cross-direction) as it passes under the lay-on roll.
When this material is run in a successive process the uneven tension caused by the material stretching leads to varying tensions across the web width. Tensile stresses are caused in the thicker areas while compress stress are present in thinner areas. This lateral compression results in the lateral shifting of the web (due to the ‘bagginess’ caused by the increased width) causing the material to overlap or bunch-up into wrinkles.
‘Tin-canning’ may not be immediately visible during or after winding. Entrapped air between layers of film may temporarily mask this defect. As the boundary air escapes over time the wraps begin to fold into the traditional MD wrinkles or “tin canning” pattern.